1. dns resolver should be global unique so dns cache can work. avoid dns query influence hole punching.
2. when system dns failed, fallback to hickory dns.
This patch implement a restful server without any auth.
usage:
```bash
# run easytier-web, which acts as an gateway and registry for all easytier-core
$> easytier-web
# run easytier-core and connect to easytier-web with a token
$> easytier-core --config-server udp://127.0.0.1:22020/fdsafdsa
# use restful api to list session
$> curl -H "Content-Type: application/json" -X GET 127.0.0.1:11211/api/v1/sessions
[{"token":"fdsafdsa","client_url":"udp://127.0.0.1:48915","machine_id":"de3f5b8f-0f2f-d9d0-fb30-a2ac8951d92f"}]%
# use restful api to run a network instance
$> curl -H "Content-Type: application/json" -X POST 127.0.0.1:11211/api/v1/network/de3f5b8f-0f2f-d9d0-fb30-a2ac8951d92f -d '{"config": "listeners = [\"udp://0.0.0.0:12344\"]"}'
# use restful api to get network instance info
$> curl -H "Content-Type: application/json" -X GET 127.0.0.1:11211/api/v1/network/de3f5b8f-0f2f-d9d0-fb30-a2ac8951d92f/65437e50-b286-4098-a624-74429f2cb839
```
the punching task creator doesn't check if the task is already
running, and may create many punching task to same peer node.
this patch also improve hole punching by checking hole punch packet
even if punch rpc is failed.
